Question 142142
The sum of the first n terms of an arithmetic progression is {{{na+(n(n-1)r)/2}}}.

n=9    <--- # of terms
a=2  <--starting term
r=-7  <--- common difference between terms


s={{{9*(2)+9(9-1)(-7)/2}}}
={{{18-252=-234}}}


We can check by iterating every term:
2-5-12-19-26-33-40-47-54=-234